The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors
As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, this switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 100,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,000 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like.
The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which are either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hoses leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank, or a charged hose, or any other charged part of the system, risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode to the grounded point as well.
One of the methods and apparatus for reducing the electrical shock problem is disclosed, for example, in U.S. Pat. No. 4,313,475 to Wiggins. In apparatus of this type, a "voltage block" system is employed wherein an electrostatically conductive coating material is first transmitted from a grounded primary coating supply into a transfer vessel which is electrically isolated from one or more electrostatic coating dispensers. After being filled with coating material, the transfer vessel is first disconnected from the primary coating supply and then connected to an inventory tank, which, in turn, is connected to the coating dispensers. The coating material is transmitted from the transfer vessel into the inventory tank, with the transfer vessel disconnected from the primary coating supply, to fill the inventory tank with coating material for subsequent transfer to the coating dispensers. After the inventory tank is filled, the transfer vessel is disconnected from the inventory tank and connected back to the primary coating supply to receive another quantity of coating material so that the coating operation can proceed essentially continuously.
Another "voltage block" system for transferring electrically conductive coating materials is disclosed in U.S. Pat. No. 5,078,168, which is owned by the assignee of this invention. In this system, first and second shuttle devices are selectively connected to two large reservoir, piston pumps. The first shuttle device is movable between a transfer position, and a spaced, neutral position, relative to a filling station which is connected to a source of electrically conductive coating material. At the filling station, the first shuttle is operative to transfer coating material from the source into the reservoir of the first pump. In the neutral position, the first shuttle is electrically isolated, i.e., physically spaced, from the filling station. The second shuttle device is movable between a transfer position wherein it interconnects the first piston pump with the second piston pump, and a neutral position wherein the two pumps are electrically isolated from one another and the second piston pump supplies coating material to the dispensers. Movement of the shuttles is controlled to maintain one of the shuttles in a neutral position while the other is at the transfer position so that there is never a completed electrical path between the source of electrically conductive coating material and the electrostatically charged dispenser.
One problem with apparatus of the type disclosed in U.S. Pat. Nos. 4,313,475 and 5,078,168 involves the pressure available to discharge the coating material from either the transfer vessel of the Wiggins apparatus or the second reservoir above the Konieczynski apparatus. For example, in the Konieczynski apparatus, each of the first and second reservoir pumps includes a piston which is movable in one direction in response to the application of air pressure thereagainst to discharge coating material from the reservoir, and is movable in the opposite direction as new coating material is added to the reservoir. In order to permit filling of the reservoir of the second pump with coating material supplied from the first pump, the air pressure applied to the piston in the second pump must be reduced compared to that of the first pump, otherwise the piston within the second pump would not move and allow the reservoir therein to be filled. Because of this reduced pressure level within the second pump, the coating material is discharged therefrom at a relatively low pressure level. As a result, a comparatively few coating dispensers can be supplied with coating material, and the spray pattern emitted from such dispensers is not always stable.
Another problem with voltage block systems of the type described above, and particularly the Konieczynski apparatus disclosed in U.S. Pat. No. 5,078,168, is a relatively wide pressure fluctuation in the coating material discharge from the second pump to the coating dispensers. When the reservoir of the second pump is filled and coating material is discharged by its piston moving in a downward direction toward the base of the reservoir, the fluid pressure output from the second pump is less than the air pressure at which the piston is forced downwardly because the seal friction with which the piston seals against the side walls of the pump reservoir opposes downward motion of the piston. This produces a comparatively low fluid discharge pressure, significantly lower than the air pressure, with the attendant disadvantages noted above. On the other hand, a higher fluid discharge pressure, e.g. higher than the air pressure, is output from the second pump when it is filled with coating material from the first pump. This is because the fluid pressure of the coating material introduced at the base of the second pump, on the bottom side of the piston, must overcome both the air pressure acting on the opposite or top side of the piston and the seal friction of the piston seals against the sidewall of the piston reservoir. Since the air pressure in the system remains constant, the fluid pressure fluctuates depending on whether the piston within the second pump is moving upwardly or downwardly. Accordingly, a potentially large pressure fluctuation can occur at the discharge side of the second pump depending upon whether or not the second pump is undergoing a fill cycle or a discharge cycle when coating material is discharged therefrom to the coating dispensers. Such pressure fluctuation limits the number of dispensers which can be supplied by the second pump, and/or adversely affects the spray pattern obtained from such dispensers.
Another problem with apparatus of the type disclosed in U.S. Pat. Nos. 4,313,475 and 5,078,168 is that an appreciable pressure drop is produced when water, solvent and/or air is used to flush the system of paint of one color in preparation for the use of another colored paint. This pressure drop occurs because, as noted above, all of the hoses and transfer containers or pumps are interconnected in series with one another from the point at which the source of coating material is introduced into the system to the point at which the coating material is discharged to the coating dispensers. For example, in the system of U.S. Pat. No. 5,078,168, the coating material, flushing liquid and/or air must first enter the lines interconnecting the first shuttle to the first pump, travel through the line interconnecting the first pump to the second pump and then pass through the lines interconnecting the second pump to the coating dispenser. By the time the flushing fluid or coating material reaches the downstream portions of this flow path, a pressure drop has occurred which lessens the effectiveness with which the air or liquid can remove the coating material remaining in the system.
While both of the systems disclosed in the Wiggins Patent No. 4,313,475 and Konieczynski Patent No. 5,078,168 are adapted for use with color changers connected to sources of different color paint, neither system is capable of effecting a color change rapidly in a production environment. Both of these systems provide an essentially "series" flow path between the source(s) of coating material and the dispensers. That is, the coating material is first transmitted from the source to the transfer vessel of the Wiggins apparatus, or to the first reservoir pump of the Konieczynski apparatus, and then delivered through lines to either the inventory tank or second reservoir pump for subsequent supply to the dispensers. In order to effect a color change in either system, a flushing liquid such as water must be introduced at the beginning of this flow path, i.e., where the coating material is introduced, and then pass through each line and element of the system in sequence, one after the other, to remove the old paint. In applications such as the coating of automobiles and/or other assembly line-type painting operations, such a relatively long "downtime" between color changes is unacceptable.